The effect of GABA on the rate of labeling of brain mitochondrial nucleotides has been determined. GABA decreases the rate of labeling of GTP. The effect of GABA on the formation of succinate from ketoglutarate by brain mitochondria was assessed by a double isotope technique. Succinate formation from GABA was found to be non-competitive with that from alpha-ketoglutarate. Together these observations suggest that the "GABA shunt" operates as a pathway independent of the flux through the conventional tricarboxylic acid cycle. The suppression of GTP turnover by GABA cannot be explained as a competitive suppression of substrate level phosphorylation.